When drilling holes through glass and similar brittle materials, it is common to drill a hole partially through the material from one side and to drill an axially aligned hole from the opposite side of the material until the holes converge. If a hole is drilled through the material from only one side, there is a risk that the material will shatter as the drill bit approaches and emerges from the opposite side. Typically, a first bit is advanced to drill a hole about half way through the material from one side and then a second bit is advanced to drill a substantially coaxial hole from the opposite side until the holes converge. For optimum performance, it is desirable to drill substantially half way through the sheet of material with the first drill and to operate the second drill only until the opposing holes converge. Because of the hardness and the brittleness of the material, the drill bits must be advanced into contact with the material at a very slow rate. If the location of the drill bit tip is not known relative to the location of the surface, the drill bit must be advanced very slowly from some distance from the surface so as not to fracture the material with a sudden impact. The location of the drill bit tip will vary from drill bit to drill bit due to manufacturing tolerance variations and there may be slight variations in the location of the surface of the material. Further, the location of the drill bit tip will change as the bit wears during use.
To facilitate drilling and to prolong the life of the bit, a flow of cooling fluid is delivered to the cutting surface of the drill bit during drilling. If the bit is of adequate diameter, the cooling fluid flows through an axial passage through the bit and discharges between the bit and the adjacent surface being drilled. While the bit is in contact with the material, the cooling fluid delivered to the bit will have a substantially higher back pressure and a lower flow rate than when the bit is spaced from the material. It is known, for example, that the back pressure of the cooling fluid can be used to determine when the bit is drilling through glass. In U.S. Pat. No. 5,028,176, for example, hollow bits are used to drill a hole in glass. After a lower bit drills part way through the glass, an upper bit drills a converging hole. When the hole portion drilled by the upper bit converges with the hole portion drilled by the lower bit and the core falls from the hole, there is a sudden drop in the cooling fluid back pressure. This fluid pressure drop is used to indicate that the hole has been completed and that drilling may be discontinued.